MESSAI.iO

AI summary

78% confidence

This review synthesizes bioelectrochemical ammonia recovery from wastewater across four BES architectures (MFC, MEC, MDC, BEC). The core mechanism couples electromigration of NH4+ across cation exchange membranes with pH-driven conversion to NH3 in alkaline cathodes, followed by extraction via stripping, transmembrane chemisorption, or precipitation. MFCs achieve net energy yields of −2.8 to −10 kJ/gN but suffer low current density (~0.02–2.6 A/m²); MECs reach higher current (up to 27 A/m²) and TAN recovery rates (70 gN/m²/d) but require external energy (4.68–35.6 kJ/gN). The paper identifies critical bottlenecks: competition from co-cations (Na+, K+, Mg2+) reducing NH4+ transport number, energy-intensive stripping (>50% of total energy), and imbalance between TAN loading rate and current generation (load ratio LN).

Key insights

  • Threshold NH4+ transport number is often <1, with competing cations (Na+, K+, Mg2+, Ca2+) reducing NH4+ selectivity; ideally LN should equal 1 to balance TAN loading and current generation.

    So what. Pretreat or dilute influent to reduce competing cations, or operate at LN ≤1 to avoid TAN accumulation in anode; monitor transport number during operation.

  • Trade-off Stripping energy consumption exceeds 50% of total MEC energy budget; TMCS and GDE-based recovery reduce this but remain underexplored at scale.

    So what. Prioritize TMCS or gas diffusion electrode integration over air-stripping for new designs; quantify aeration energy as a design constraint.

  • Trade-off Cathode pH reaches 13.8 at 30 µm from surface at ~17 A/m² current density, driving NH4+ → NH3 conversion, but this pH gradient also increases overpotentials and reduces energetic efficiency.

    So what. Operate at moderate current density (1–3 A/m²) to balance NH3 formation against overpotential losses; use pH buffering or recirculation to flatten gradients.

  • Trade-off MFCs achieve negative energy consumption (−2.8 to −10 kJ/gN) but only at very low current density (0.02–2.6 A/m²), limiting TAN recovery rate to <10 gN/m²/d; MECs reach 70 gN/m²/d but require +4.68–35.6 kJ/gN input.

    So what. Choose MFC for energy-positive recovery of dilute streams (<1 g/L TAN); choose MEC for high-strength waste (>2 g/L) where energy cost is acceptable and throughput matters.

  • Performance Triple-chamber MECs with TMCS or GDE achieve 66–89% TAN recovery efficiency at 1.4–3.6 A/m² with lower energy (5.79–18 kJ/gN) than double-chamber stripping systems.

    So what. Adopt triple-chamber architecture with membrane contactors for new pilot designs; prioritize GDE cathodes to enable high current and low stripping energy.

Contradictions1

Tensions the paper itself points to. Each side quotes the passage it comes from.

  • Within the paper NH4+ transport dominance across CEM is disputed: one study claims 100% transport per electron, another claims only 40%.

    One side

    one NH4+ ion transported per electron passing through external circuit

    “some studies indicate that NH4+ transport dominates the current across the CEM and one NH4+ion is transported per each electron passing through the external circuit”
    Other side

    NH4+ ions may transport only 40% of total current

    “other studies suggest that NH4+ ions may transport only 40% of the total current”

    Authors’ reading. Authors note the link between NH4+ transport and current density is 'not fully understood yet'; difference likely depends on competing cation concentration, membrane type, and operating conditions. No reconciliation offered.

Opportunity areas6

Specific gaps in this paper, with an experiment that would close each.

  • High No systematic study of load ratio (LN) optimization across current density and TAN loading ranges; paper defines LN but does not report how to operate at LN=1 or explore LN >1 regimes.

    Section 2.1 introduces LN as critical (LN<1 → TAN accumulation; LN=1 → balanced; LN>1 → excess current) but no experimental data vary LN systematically. All reported studies operate at unknown LN.

    Try. Vary influent TAN concentration (0.1–5 g/L) and applied current (0.5–20 A/m²) in a double-chamber MEC; measure TAN accumulation in anode and transport number; identify LN=1 operating window.

  • High Competing cation effects (Na+, K+, Mg2+, Ca2+) on NH4+ transport number are mentioned but not quantified; no study reports transport number as a function of competing cation concentration or ratio.

    Section 2.1 states 'presence of cations other than NH4+ in high concentration can hinder TAN recovery' and cites transport number (Eq. 4) but no table or figure shows how transport number varies with Na+/NH4+ or K+/NH4+ ratio.

    Try. Prepare synthetic wastewaters with fixed TAN (1 g/L) and variable Na+/K+/Mg2+ (0–5 g/L each); measure transport number in double-chamber MEC at 2 A/m²; plot transport number vs. competing cation concentration.

  • High Cathode potential and overpotential are not reported in most studies; no data on how cathode potential (vs. SHE or Ag/AgCl) varies with current density, pH, or recovery method.

    Section 2.2 calculates pH at electrode surface but does not report measured cathode potentials. Tables 1 and 2 list current density and energy consumption but not cathode potential, making it impossible to assess overpotential losses or compare systems.

    Try. Operate double-chamber and triple-chamber MECs at 1, 5, 10, 20 A/m² with reference electrodes (Ag/AgCl); measure cathode potential, anode potential, and cell voltage; calculate overpotentials and energy efficiency.

  • Medium N2O or NO2− accumulation is not mentioned; no study reports whether denitrification or nitrification side-reactions occur at anode or cathode, or whether N2O is produced during recovery.

    Paper focuses on NH4+ recovery but does not address nitrogen speciation at electrodes. Cathode pH >13 and high current could drive unintended reactions (e.g., NO3− reduction to N2O). No gas chromatography or ion chromatography data for N-species.

    Try. Operate MEC with 1 g/L TAN + 0.5 g/L NO3− at 5 A/m²; sample cathode gas and catholyte at 1, 6, 24 h; measure N2O, NO, NO2−, NH3 by GC-MS and IC; quantify N balance.

  • Medium Hydraulic retention time (HRT) and residence time effects on TAN recovery are not systematically studied; most lab studies use batch or fed-batch mode with undefined HRT.

    Tables 1 and 2 do not report HRT or flow rate; paper mentions 'inlet ammonium concentration' as a bottleneck but does not explore how TAN RR scales with HRT or space velocity.

    Try. Operate a continuous-flow double-chamber MEC at 2 A/m² with 1 g/L TAN influent; vary HRT (2, 6, 12, 24 h) by adjusting flow rate; measure TAN RR, TAN RE, and energy consumption vs. HRT.

  • Medium Membrane fouling and cation selectivity degradation over time are not reported; no long-term (>30 day) study of CEM performance or transport number drift.

    Section 2.1 reviews CEM properties but all cited studies are short-term (<7 days typical). Paper identifies 'use of membranes' as a critical challenge but provides no data on fouling, ion exchange capacity loss, or selectivity decay.

    Try. Operate a double-chamber MEC with Nafion N117 CEM for 60 days at 2 A/m² with 1 g/L TAN + 2 g/L Na+; measure transport number weekly; post-test SEM/EDS of membrane; quantify capacity loss.

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What they did

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REVIEW

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Abstract

Due to the anticipated rise in demand for ammonia, the search for viable methods to recover it has grown in recent years, as traditional ammonia production is a high energy intensive process. Among the different, bioelectrochemical systems (BESs) offer an alternative solution for ammonia recovery since they have shown lower energy demand (in terms of kJ g−1N recovered) at lab-scale than other methodologies. In BESs, the bioelectrochemically generated current drives the transport of NH4+ from the influent to a concentration chamber through a cation exchange membrane before its subsequent recovery. This paper describes the fundamentals and opportunities for bioelectrochemical ammonia recovery (either by stripping. absorption or precipitation) in different BES devices such as microbial fuel cells, microbial electrolysis cells, microbial desalination cells and bioelectroconcentration cells and compares the performance of all the reported experimental works so far. Moreover, the most critical challenges for its scale-up (low current density, nature and quantity of the carbon source, inlet ammonium concentration, use of membranes, energy yield and recovery efficiency) have been detailed and discussed in view of better understanding the current bottlenecks for its scale-up and, thus, for its prompt industrial adoption.

Keywords

DesalinationAmmoniaWastewaterElectrolysisMicrobial electrolysis cellMicrobial fuel cellEnvironmental scienceEnergy recoveryProcess engineeringSewage treatmentAmmoniumStripping (fiber)Waste managementPulp and paper industryChemistryEnvironmental engineeringBiochemical engineeringMembraneMaterials scienceElectrolyteEngineeringEnergy (signal processing)Anode

Identifiers

DOI
10.1016/j.cej.2023.144855
Journal
Chemical Engineering Journal
Year
2023
Extraction

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